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Adds a target for the library and test targets.

- Integrate into compontents_unittests as all other components do as
  well. This ensures that unittests are automatically run on CI.
- Provide a separate gc_unittests binary as the components binary
  requires ~35k files to build.

Bug: 1056170
Change-Id: I8fa235c239f2dc5217145c12abb8c1f27af84fc5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2080422
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745597}